Implantable medication delivery device

ABSTRACT

An implantable medication delivery device that is highly space efficient and can reliably and safely deliver controlled medication doses to a target site. The system includes a variable volume medication reservoir that is exposed to an ambient pressure equal to the ambient pressure at a system outlet port. A pump/valve subassembly is provided to draw medication from the reservoir and force a medication dose along a fluid transfer passageway to the outlet port. The pump/valve subassembly incorporates a safety mechanism, e.g., a balanced valve, which normally blocks medication flow to the outlet port and opens only in response to a pump induced unbalancing force. A protective shell is also disclosed for protecting the ambient reservoir.

RELATED APPLICATIONS

This application claims the benefit of U.S. Application 60/383,237 filedon 22 May 2002.

FIELD OF THE INVENTION

The present invention is directed to implantable medication deliverydevices useful for delivering prescribed medication doses to targetedbody sites.

BACKGROUND OF THE INVENTION

Commercially available implantable medication delivery devices areexemplified by a Synchromed product marketed by Medtronic (ofMinneapolis, Minn., USA) and a MIP product manufactured by Minimed, nowa division of Medtronic. Both of these devices employ a medicationreservoir comprising a bellows that contracts as medication is extractedby a pump mechanism. The reservoir volume change is accommodated by asecond chamber which contains a propellant such as Freon in a gas/liquidequilibrium. The propellant functions to maintain a constant absolutepressure at body temperature. In the case of the MIP product, thepropellant is a liquid at body temperature creating a negative pressurereservoir. In the case of the Syncromed product, the propellant is a gasat body temperature creating a positive pressure reservoir. In bothcases, the medication reservoir is maintained at a constant absolutepressure by the propellant. Although the reservoir, and therefore theinlet side of the pump mechanism are at a constant absolute pressure,the tip of an output catheter and thus the outlet of the pump mechanism,are at ambient pressure. Ambient pressure typically varies as a functionof environmental conditions including local barometric pressure andaltitude, etc. In addition, variations in temperature can producevariations in reservoir pressure. The combined effect of theseconditions can produce pressure differences in excess of 500 millibarsacross the pump mechanism. In order to seal and pump across a pressuredifference of this magnitude, these exemplary systems require pumps of asize which are not well suited for implantation in space limited sites,e.g., the brain, eye, or ear.

SUMMARY OF THE INVENTION

The present invention is directed to a medication delivery devicecomprising an ambient reservoir and housing integrated so as to behighly space efficient for reliably and safely delivering controlledmedication doses to a target body site. Devices in accordance with theinvention include a mounting structure for supporting a reservoirperipheral wall which includes a movable, e.g., flexible, portion. Thereservoir wall has an outer surface exposed to an ambient pressure(equal to the pressure at the tip of an output catheter) whichestablishes the same pressure within the reservoir interior volume. As aconsequence of the reservoir and catheter tip being at the samepressure, the pump size and energy requirements are reduced as comparedto the aforementioned exemplary prior art systems.

In accordance with the invention, the reservoir wall encloses a variablevolume for storing medication. The movable reservoir wall portion can beformed of flaccid nonextensible nonporous material or, alternatively,can be formed by a bellows or telescoping tubular sections. The mountingstructure for supporting the reservoir wall preferably incorporates apump/valve subassembly operable to draw medication from the reservoirvia a fluid inlet and force medication along a fluid transfer passagewayto an outlet port adapted to be coupled to the output catheter.

In order to reliably use an ambient pressure reservoir, a device inaccordance with the invention is configured to prevent medicationleakage (or flowthrough), i.e., unintended medication discharge throughthe outlet port, as a result of reservoir overfill and/or a pressure orforce being applied to the reservoir. More particularly, it isunacceptable for medication to be discharged as a result of thereservoir wall being pressed, e.g., as a consequence of the patientbeing bumped. Thus, in accordance with a first preferred embodiment, theaforementioned pump/valve subassembly incorporates a safety mechanismwhich functions to normally block unintended fluid flow to the outletport. One preferred safety mechanism in accordance with the inventionuses a balanced valve which responds to a difference between thereservoir pressure and a pump chamber pressure. That is, an increase inreservoir pressure acts in a direction to seal closed the safetymechanism valve whereas an increase in pump chamber pressure acts toopen the valve to effectively disable its normal blocking function.

In accordance with an alternative and/or additional feature forpreventing medication flowthrough, a protective substantially rigidshell is mounted around the reservoir wall to prevent the inadvertentapplication of a force thereto. In order to maintain ambient pressure inthe reservoir, the shell is configured to allow body fluid to enter andexit the shell to enable the reservoir to expand (when being filled withmedication) and contract (as medication is being discharged). Inaccordance with a second preferred embodiment, the shell includes adiffusive membrane (e.g., cellulose acetate membrane) that permits bodyfluid to flow slowly into the shell interior volume but preventsundesirable tissue growth therein. The shell preferably also includes acheck valve which permits relatively rapid fluid outflow to permit thereservoir to fill and expand within the shell interior volume.

A preferred mounting structure in accordance with the invention supportsthe reservoir wall and functionally integrates the pump/valvesubassembly. The pump/valve subassembly includes an inlet port and afluid passageway extending to an outlet port. A first check valve,permitting fluid inflow only is included in the passageway downstreamfrom the fluid inlet. A pump chamber is included in the passagewaybetween the first check valve and a safety mechanism located upstreamfrom the outlet port. A pump element coupled to the pump chamber isoperable to produce (1) a suction for drawing medication past the firstcheck valve into the pump chamber and (2) a pressure for expellingmedication from the pump chamber toward said safety mechanism.

As previously mentioned, the safety mechanism is provided to preventunintended medication flow to the outlet port. The safety mechanismincludes a valve element movable between (1) a flow position and (2) aflow-block position. In a preferred embodiment, the safety valve elementis normally in the flow-block position. However, a pressure increase inthe pump chamber produced by the pump element acts to move the safetyvalve element to the flow position thus temporarily disabling the flowblocking function.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic block diagram of a basic ambient pressuremedication delivery system;

FIG. 2 is a schematic block diagram similar to FIG. 1 modified torepresent two alternative features for preventing medication dischargeas a consequence of a force applied to the medication reservoir, namely(1) a safety mechanism responsive to an applied force for blocking flowto the output port and (2) a protective shell mounted around thereservoir;

FIG. 3 is an isometric exterior view of a preferred implantablemedication delivery system in accordance with the present invention;

FIG. 4 is a cross-sectional view showing the system of FIG. 3 implantedat an exemplary site in a patient's body for infusing medication intothe patient's brain;

FIG. 5 is an exterior side view of a preferred embodiment showingexemplary dimensions;

FIG. 6 is a horizontal sectional view taken substantially along theplane 6-6 of FIG. 5;

FIG. 7 is a vertical sectional view taken substantially along the plane7-7 of FIG. 6;

FIG. 8 is an enlarged sectional view showing a preferred pump/valveassembly in accordance with a first embodiment of the invention;

FIG. 9 is a plan view of a second embodiment of the inventionincorporating a protective shell around the reservoir;

FIG. 10 is a sectional view taken substantially along the plane 10-10 ofFIG. 9; and

FIG. 11 is a sectional view of an alternative preferred embodiment ofthis invention.

DETAILED DESCRIPTION

Attention is initially directed to FIG. 1 which schematically depicts animplantable medication delivery system 20 including a variable volumereservoir 24 for storing medication. The reservoir 24 is preferablyrefillable, e.g., via a fill device 26 and tube 28 coupled to areservoir fill port 30. The fill device 26 preferably defines a conicalentrance 32 for guiding the needle of a syringe (not shown) through aself healing septum 34 to a channel 36 and check valve 38. The outlet ofcheck valve 38 is coupled via nipple 40 to the upstream end of tube 28.The downstream end of tube 28 is coupled to the reservoir fill port 30.

The variable volume reservoir 24 is comprised of a wall 42 including atleast a portion supported for movement to enable the reservoir interiorvolume 44 to expand and contract. Although the reservoir 24 is mostsimply formed of flexible, or flaccid, nonextensible nonporous materialforming a sack, it can also be provided in various alternativeconfigurations. For example, the reservoir 24 can be configured as abellows, telescoping tubular sections, or as a shaped rubber boot havinga stiffened base such that the base lifts and the boot's sidewall rollsupon itself, as the reservoir interior volume changes.

The reservoir outlet 46 is coupled via a fluid passageway 48 to a systemoutput port 50. The system output port 50 is typically coupled to acatheter 52 whose downstream end, or tip 54, is intended to infusemedication into targeted body tissue, e.g., brain tissue, blood orintraperitoneal space. The fluid passageway 48 is comprised of a first,or upstream, check valve 56 which leads to an entrance port 58 of a pumpchamber 60. A pump chamber exit port 62 is coupled to a second, ordownstream, check valve 64 which leads to the aforementioned systemoutput port 50.

The pump chamber 60 is defined by a peripheral wall 68 including amovable portion, e.g., a piston or diaphragm 70. The diaphragm 70 iscoupled to an actuator 72 configured to displace the diaphragm 70reciprocally between a first position which contracts the volume of thepump chamber 60 and a second position which expands the volume of thechamber 60. Thus, when the diaphragm 70 moves downwardly as representedin FIG. 1, the pump chamber 60 will expand in volume creating a negativepressure which draws medication from the interior reservoir volume 44past the check valve 56 into the pump chamber 60. On the other hand,when the diaphragm 70 moves upwardly, as represented in FIG. 1, aportion, or dose, of the medication in chamber 60 will be expelledthrough exit port 62 past check valve 64 to the system output port 50.

A system of the type represented in FIG. 1 is intended in accordancewith the invention to be implanted in a patient's body such that thereservoir wall 42 and the catheter tip 54 are both exposed tosubstantially the same ambient pressure. An isometric exterior view ofan exemplary embodiment 80 in accordance with the invention is shown inFIG. 3. Note that the embodiment 80 includes a housing 82 carrying anintegrated reservoir 84, analogous to the aforedisccussed reservoir 24,of FIG. 1. Also note that a catheter tube 86, analogous to theaforementioned catheter tube 52, extends outwardly from the housing 82.Further note that a fill device 88, analogous to aforedisccussed filldevice 26, is coupled to the housing 82 via a fill tube 90.

FIG. 4 depicts an exemplary embodiment 80 implanted in a patient's bodyin accordance with one significant application of the invention as acranial pump for delivering medication to brain tissue. Embodiments ofthe invention can be advantageously used in a variety of otherapplications, e.g., eye, ear, brain. Note that FIG. 4 represents apatient's skull at 100 covered by a patient's skin 102 and hair 104. Inthe contemplated implant procedure for the embodiment 80, a recess 106is surgically formed in the patient's skull for accommodating thehousing 82. The reservoir portion 84 of the embodiment 80 lies beneaththe skin 102 as depicted. The fill device 88 is also shown as beingsubcutaneously implanted. As is well known, the subcutaneous fill device88 can be used together with a syringe to fill reservoir 84 with fluidmedication. FIG. 4 also depicts the output catheter 86 extending fromthe device 80 with the catheter tip 87 positioned to infuse medicationinto the patient's brain.

In order to use an ambient pressure reservoir in the medication deliverysystem exemplified by FIG. 1 and implanted in the exemplary manner shownin FIG. 4, various problems have to be addressed to insure patientsafety and device reliability. One such problem is to prevent medicationdischarge from catheter tip 87 as a consequence of inadvertentlyoverfilling the reservoir and/or unintentionally applying a force to thereservoir wall 42. That is, it is important in accordance with theinvention to prevent flowthrough, i.e., an unintended delivery ofmedication as a consequence, for example, of a physician overpressurizing the reservoir by introducing too much medication and/or thepatient being bumped or pressing the reservoir wall.

Thus, preferred embodiments of the invention, as detailed in FIGS. 5-11,incorporate (1) a safety mechanism for preventing medication flowthroughin the event of a pressure increase in the reservoir, e.g., attributableto a force exerted against the reservoir wall and/or (2) a protectiveshell around the reservoir to prevent the inadvertent application of aforce to the reservoir wall. FIG. 2 depicts a modification of theambient pressure reservoir medication delivery system of FIG. 1 to showthe inclusion of an exemplary (1) safety mechanism 110 responsive toreservoir pressure via channel 111 and (2) protective reservoir shell112. The safety mechanism 110 and protective shell 112 can be usedseparately or in combination.

Attention is now directed to FIGS. 5-8 which show the details of a firstembodiment of the invention, consistent with the exterior representationshown in FIGS. 3 and 4, and incorporating the safety mechanism 110 ofFIG. 2. FIG. 5 comprises an exterior side view of the embodimentdepicted in FIG. 3 showing exemplary dimensions (inches) for presentlycontemplated implant applications. The following table shows exemplaryspecifications for the applications indicated: Very Small Pump; Very LowDelivery Rate Small Pump; Low Delivery Rate Parameter Typical Min MaxTypical Min Max Medication 3 0.5 5 20 5 40 Reservoir Volume (ml) DailyDelivery Rate 0.05 0.03 0.1 0.33 0.1 0.66 (ml/day) Maximum Delivery 3 110 30 5 120 Rate (ul/minute) Stroke Volume 0.2 0.05 1 0.5 0.1 5(microliters) Maximum Output 14.7 7 100 14.7 7 100 Pressure (psig)Longevity (years) 8 3 10 8 3 10 Refill Interval (days) 60 30 90 60 30 90Application (typical) Tinnitus using lidocaine Pain using morphine;Spasticity (CP) using baclofen Route of Delivery Intracranial, eye, earIntrathecal, epidural, Intraperitoneal, systemic

The medication delivery device depicted in FIGS. 5-8 is comprised of ahousing 120 formed by a bowl-shaped wall 122 that includes ahorizontally oriented (as viewed in FIG. 7) circular base 124 having acylindrical side wall 126 extending vertically therefrom. The upper edgeof sidewall 126 flairs radially outward to form a horizontal flange 128.A circular partition plate 130 is supported above the base 124 to form aclosed compartment for housing a battery 132 (preferably remotelychargeable). The upper surface of partition plate 130 is preferably usedto support an electronic control module 136 and an electrically drivenpump actuator 138, analogous to the aforementioned actuator 72. Asubstantially planar pump/valve subassembly 140 is supported on theupper surface of flange 128 above the module 136 and pump actuator 138.

The housing 120 and subassembly 140 together form a mounting structurefor supporting a reservoir wall 144 of nonextensible nonporous materialwhich extends loosely over the subassembly 140. The peripheral edge 146of wall 144 is preferably sealed to the under surface of flange 128 tothus form a closed reservoir volume 148 above the upper surface ofsubassembly 140 for storing fluid medication.

The pump/valve subassembly 140 preferably comprises a thin flatstructure formed by laminating two or more plates 152, 154. Thelaminated plates can be formed and assembled using a variety ofmaterials, e.g., titanium, stainless steel, silicon, plastic, etc. andknown fabrication techniques appropriate to the materials and thedesired dimensions and tolerances.

With continuing reference to FIG. 8, note that upper plate 152 definesan inlet port 160 of a fluid passageway 161 leading to an outlet port164. The fluid passageway 161 includes a first check valve 166, locatedjust downstream form inlet port 160. Check valve 166 is preferablycomprised of spring 170 that normally seals a precision ball 172 againstvalve seat 174. The outlet of check valve 166 opens via port 175 to pumpchamber 176 whose peripheral wall is defined in part by flexiblediaphragm 178. The pump chamber outlet 179 leads to the inlet of asecond check valve 180 preferably comprised of spring 182 normallysealing ball 184 against valve seat 186.

The pump diaphragm 178 is mounted for movement, as by coupling it to astem 188 of the linear actuator 138. When the actuator 138 pulls thestem downward (as viewed in FIG. 8) to increase the volume of pumpchamber 176, the resulting suction draws medication past check valve 166into the pump chamber 176. When the actuator 138 drives the stem 188upward, the diaphragm 178 produces a positive pressure to expelmedication from the pump chamber 176 past the second check valve 180toward the outlet port 164. For simplicity of explanation herein, theactuator has been described as pulling the stem downward and driving thestem upward. It should be understood however, that the diaphragm could,in fact, be biased to one position so that the actuator need only moveit from the biased position

The outlet of check valve 180 opens via port 194 to safety valve 196,analogous to safety mechanism 110 of FIG. 2. Safety valve 196 ispreferably comprised of a flexible, e.g., elastomeric, valve disc 198mounted so that its upper surface normally seals against the valve seat200 (flow-block position) in the absence of a force produced by anupward stroke of diaphragm 178. More particularly, When the actuator 138is dormant, the upper face of valve disc 198 is exposed to ambientreservoir pressure via check valves 166 and 168. The lower face of valvedisc 198 is also exposed to ambient reservoir pressure via channel 204,analogous to channel 111 of FIG. 2. Parenthetically, note also thatchannel 204 defines a path from fill nipple 206, analogous to input 30of FIG. 1, to the reservoir volume interior 148.

Under normal conditions with the actuator 138 dormant, ambient reservoirpressure is applied to both faces of valve disc 198 and it remains in aflow-block position sealed against valve seat 200 so as to block outflowfrom check valve 180 to output port 164. If the reservoir pressureincreases, e.g., attributable to the patient being bumped or pressingthe reservoir wall, the pressure will increase equally on both faces ofthe valve disc 198, thereby leaving the disc 198 seated. Thus, theinclusion of safety valve 196 upstream from outlet port 164 prevents afailure mode which could, in the absence of the safety valve,unintentionally force medication out through the outlet port 164. Whenthe actuator 138 is activated, however, the upward movement of diaphragm178 forces medication from the pump chamber 176 past the check valve 180to the upper face of valve disc 198. The resulting unbalanced pressureon valve disc 198 unseats the disc thereby disabling its flow blockingfunction to permit medication to flow therepast to the outlet port 164.

Attention is now directed to FIGS. 9 and 10 which illustrate a preferredprotective shell 240 (corresponding to shell 112 of FIG. 2) configuredto protect the reservoir wall 144 from impact, while still exposing itto ambient pressure and allowing it to expand and contract. The shell240 preferably comprises a dome-shaped rigid or semi-rigid frame 242including a hub 244 and radial arms 246 extending to an outer ring 248.The outer ring 248 carries inwardly extending flange portions 250configured to mount around housing flange 128. The shell 240 is shapedand dimensioned to define an interior volume 252 able to snuglyaccommodate reservoir wall 144 in its fully expanded state.

In order to expose the reservoir wall 144 to ambient pressure and permitit to expand and contract within the shell interior volume 252, meansare provided to allow body fluid to enter into and exit from theinterior volume 252. More particularly, a diffusive membrane 258preferably formed of a cellulose acetate or similar material, is mountedbetween the hub 244 and outer ring 248. The diffusive membrane materialis preferably selected to permit slow diffusion of body fluids into thevolume 252 while preventing the in-growth of body tissue. A slow rate offluid inflow is acceptable because, in typical applications, thereservoir will contract at a maximum rate of only about 40 millilitersper month.

On the other hand, when the reservoir is refilled via fill nipple 206, agreater rate of outflow from the volume 252 is required. Accordingly, anoutflow check valve 264 is preferably mounted in the hub 244 to allowthe reservoir to expand relatively rapidly and force fluid out of thevolume 252. Check valve 264 is comprised of a stem 266 carrying aretention rod 268 on its lower end and a sealing disc 270 on its upperend. When the reservoir expands, it increases the pressure in volume 252to lift disk 270 permitting the outflow of fluid through opening 272around stem 266.

Attention is now directed to FIG. 11 which schematically illustrates analternative preferred medication delivery device 300 in accordance withthe present invention. The device 300 is comprised of a housing 302including a top cover plate 304 and a bottom plate 306. The spacedplates 304, 306 define an interior compartment 308 for housing a battery310, electronics 312, and a pump 314. The housing 302 and plate 306 forma mounting structure for supporting a flexible membrane 316. Themembrane 316 preferably comprises flaccid nonextensible nonporousmaterial which acts as a peripheral wall 318 enclosing an interiorreservoir volume 320.

The spaced plates 304, 306 support a reservoir fill port 324 whichincludes a self healing septum 326. The reservoir volume 320 can befilled by a hypodermic needle (not shown) penetrating the septum 326 anddischarging medication through chamber 328 and ports 329 formed in plate306.

The plate 306 functions as part of a pump/valve subassembly 330 whichincludes a fluid transfer passageway for coupling reservoir volume 320to outlet port 332. More particularly, plate 306 defines inlet port 336opening via check valve 338 into pump chamber 340. Pump chamber 340exits past outlet check valve 342 to safety valve 348. Safety valve 348includes a valve element or diaphragm 350 having one face 352 exposedvia port 354 to the pressure in reservoir volume 320. A second face 354of diaphragm 350 is exposed via check valve 342 to the pressure producedin pump chamber 340.

When the reservoir pressure exceeds the pump chamber pressure, it forcesdiaphragm 350 in a direction to seal against valve seat 360 to therebyblock unintended fluid flow from the reservoir to the outlet port 332.On the other hand, when it is intended to flow fluid from the reservoirto the outlet port, the pump chamber pressure is increased to unseatdiaphragm 350. When diaphragm 350 is unseated, medication is able toflow through the passageway from the pump chamber 340 to the outlet port332.

From the foregoing, it should now be apparent that an implantableambient pressure medication delivery system has been described includingmeans for preventing the unintended discharge of medication into thepatient's body. The described means includes a safety mechanism depictedprimarily in the embodiments of FIGS. 5-8 and 11 and a protective shelldepicted primarily in FIGS. 9 and 10. Although distinctly discussed, itshould be understood that these two techniques can be employedseparately or in combination.

It should also be understood that although specific implementations havebeen described herein, it is recognized that variations andmodifications will occur to those skilled in the art coming within thespirit and intended scope of the invention. Thus, for example only, itis pointed out that the check valves and safety valve illustrated couldtake many alternative forms using different valve elements and differentmechanisms for producing the seating force, e.g., magnetic.

1. An integrated medication delivery device configured to be implantedinto a patient's body, said device comprising: a mounting structure; amovable wall carried by said mounting structure and defining a variablevolume reservoir; a fill port for supplying medication to saidreservoir; said mounting structure defining a fluid inlet portcommunicating with said reservoir, a fluid outlet port for dischargingmedication into said body, and a fluid path operable to transfermedication from said inlet port to said outlet port; said fluid pathincluding a pump chamber, an inlet valve located between said inlet portand said pump chamber, and a safety valve located between said pumpchamber and said outlet port for preventing unintended medicationdischarge through said outlet port; a pump actuator associated with saidpump chamber operable to define a suction mode for pulling fluid viasaid inlet port from said reservoir into said pump chamber and apressure mode for forcing fluid from said pump chamber toward saidsafety valve; and wherein said safety valve permits fluid flow when thepressure in said pump chamber exceeds the pressure in said reservoir andprevents fluid flow when the pressure in said reservoir exceeds thepressure in said pump chamber.
 2. The device of claim 1 configured sothat when implanted in a patient's body, both said movable wall and saidoutlet port are exposed to ambient pressure; and wherein said safetyvalve permits fluid flow during said pressure mode when said pumpactuator acts to increase pressure in said pump chamber.
 3. The deviceof claim 1 wherein said movable wall is comprised of a flexible nonextensible non porous material.
 4. The device of claim 1 wherein saidinlet valve comprises a check valve configured to open in response tothe pressure in said pump chamber decreasing below the pressure in saidreservoir.
 5. The device of claim 4 including spring means normallybiasing said inlet valve closed.
 6. The device of claim 1 wherein saidsafety valve includes a valve element movable between open and closedpositions and including means biasing said valve element to said closedposition to normally prevent fluid flow to said outlet port; and meansfor moving said safety valve element to said open position when saidpump actuator increases pressure in said pump chamber.
 7. The device ofclaim 6 including means responsive to an increase in pressure in saidreservoir for increasing the sealing pressure on said safety valveelement to prevent fluid flow to said outlet port.
 8. The device ofclaim 1 wherein said mounting structure comprises a substantially planarstructure.
 9. An integrated medication delivery device configured forimplantation in a patient's body, said device comprising: a pump-valvesubassembly; and a movable reservoir wall mounted adjacent to saidsubassembly for defining a variable volume medication reservoir; saidpump/valve subassembly defining: a fluid inlet port open to saidreservoir; a fluid outlet port for delivering medication to a body site;and a fluid passageway operable to transfer medication from said fluidinlet port to said fluid outlet port; said fluid passageway including aninlet valve coupled to said fluid inlet port, a pump chamber locateddownstream from said inlet valve, an outlet valve located downstreamfrom said pump chamber, and a safety device coupled between said outletvalve and said fluid outlet port; an electrically operated pumpassociated with said pump chamber operable in a suction mode to pullfluid from said reservoir past said inlet valve into said pump chamberand a pressure mode to force fluid from said pump chamber past saidoutlet valve to said safety device; said safety device being operable ina first state for blocking fluid flow from said outlet valve to saidoutlet port and a second state for permitting fluid flow from saidoutlet valve to said outlet port; and wherein said safety deviceoperates in said second state when said pump defines said pressure mode.10. The device of claim 9 configured so that when implanted in apatient's body, said reservoir and said outlet port are exposed to acommon ambient pressure.
 11. The device of claim 9 wherein said safetydevice includes a valve element movable between a seated position forblocking fluid flow to said outlet port and an unseated position forpermitting fluid flow to said outlet port; and wherein said valveelement is in said seated position when the pressure in said reservoirexceeds the pressure in said pump chamber.
 12. The device of claim 11further including a fill port in communication with said reservoir forfilling said reservoir with fluid medication; and means responsive to anincrease in reservoir pressure attributable to filling said reservoirfor forcing said valve element toward said seated position.
 13. Thedevice of claim 9 wherein said inlet valve comprises a spring urgedcheck valve configured to open when the pressure in said reservoirexceeds the pressure in said pump chamber.
 14. The device of claim 9wherein said outlet valve comprises a spring urged check valveconfigured to open when the pressure in said pump chamber exceeds thepressure at said outlet port.
 15. The device of claim 9 wherein saidreservoir wall is comprised of a flexible nonporous and nonextensiblematerial.
 16. The device of claim 9 further including a protective shellmounted around said reservoir wall.
 17. The device of claim 16 whereinsaid protective shell includes a fluid inflow mechanism and a fluidoutflow mechanism for transferring ambient pressure to said reservoirwall.
 18. A medication delivery device for implanting into a patient'sbody, said device comprising: a reservoir for storing medicationcomprising a peripheral wall including a portion exposed to ambientpressure and configured for movement to vary the interior volume of saidreservoir; an outlet port exposed to ambient pressure for dischargingmedication from said device for infusion into said patient's body; and apump/valve subassembly for transferring medication at a prescribed ratefrom said reservoir to said outlet port, said subassembly including: afluid transfer passageway extending from a fluid inlet open to saidreservoir to said outlet port; a safety device operable to prevent thetransfer of medication along said passageway to said outlet port; and anactuatable pump for disabling said safety device to draw medication fromsaid reservoir and force said medication along said passageway to saidoutlet port.
 19. The device of claim 18 wherein said reservoirperipheral wall is formed of a flexible nonextensible nonporousmaterial;
 20. The device of claim 19 including a protective shellmounted around said reservoir peripheral wall; and wherein saidprotective shell includes fluid inflow and outflow means enablingambient pressure to bear against said reservoir peripheral wall.
 21. Amedication delivery device for implanting into a patient's body, saidsystem comprising: a substantially rigid protective shell defining aninterior volume, said shell including a fluid inflow mechanism and afluid outflow mechanism; a reservoir for storing medication supported insaid interior volume, said reservoir comprising a peripheral wallincluding a portion exposed to ambient pressure and configured formovement to vary the interior volume of said reservoir; an outlet portfor discharging medication from said reservoir for infusion into saidpatient's body; and a pump/valve subassembly for transferring medicationat a prescribed rate from said reservoir to said outlet port, saidsubassembly including: a fluid transfer passageway extending from afluid inlet open to said reservoir to said outlet port; and a pumpactuatable for drawing medication from said reservoir and forcing saidmedication along said passageway to said outlet port.
 22. The device ofclaim 21 wherein said reservoir peripheral wall comprises a piece offlexible nonextensible nonporous material.
 23. The device of claim 21wherein said subassembly further includes a normally closed safety valvemounted upstream from said outlet port for preventing fluid flow to saidoutlet port.
 24. The device of claim 23 including means for opening saidnormally closed safety valve when said pump is actuated to forcemedication to said outlet port.